The present invention relates to a stretch assembly used in stretching preforms in a blow molding machine and in particular to a stretch assembly driven by a servo motor.
In the production of bottles by blow molding an injection molded preform, it is often necessary to stretch the preforms longitudinally at the time of or immediately before blow molding. Such stretching is necessary to produce the needed molecular orientation in the resin. Longitudinal stretching of preforms is accomplished by inserting a stretch rod into the open end of each preform and stretching the preform by pushing longitudinally on the preform base. Such stretching is typically performed by a hydraulic or pneumatic actuating cylinder coupled to the stretch rod for moving the rod in and out of the preform.
In the mass production of bottles, it is desired that the bottles be uniform to the greatest extent possible. With the use of an actuating cylinder for moving the stretch rod, the desired uniformity is not always possible and certain limits exist in the ability to control the stretch rod movement. For example, the speed at which the cylinder and a stretch rod move, for a given operating pressure, depends upon the resistance met by the stretch rod. The resistance to stretching will vary with many factors including the temperature of the preform and the number of preforms being stretched. If one or more preforms have been dropped and only a portion of the total compliment is at the blowing station, the resistance to travel will be reduced, increasing the speed of stretching. Furthermore, with an actuating cylinder, it is difficult if not impossible to vary the stretch rate during stretching or to start and stop the motion of the stretch rods during the stretch stroke in an accurate and repeatable manner.
Accordingly, it is an object of the present invention to provide a stretch assembly that is controllable to vary the speed of travel of the stretch rod as well as to accurately control the start and stop of stretch rod movement to provide uniform preform stretching to produce containers having greater uniformity.
It is a feature of the present invention to drive the stretch assembly with a servo motor that is electronically controlled and programmable for the purpose of starting, stopping and controlling the speed of the stretch rod travel.
It is a feature of the servo driven stretch assembly that cam actuated valves on the stretch stroke can be utilized for activating the flow of blow air in an accurate and repeatable manner. Furthermore, electronic controls based on the servo location can be used in place of the cam actuated mechanical valves if desired.
It is an advantage of the servo control stretch assembly of the present invention that prior to the beginning of preform stretching, the stretch rod can be preset in position within the preform, immediately adjacent to the preform base during the process of closing the blow mold assembly, thus reducing the total cycle time by performing multiple operations at one time.
It is a further advantage that customized stretch profiles can be used for different containers.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.